Create all possible numeric combinations given n values-CodePudding

I want to create a function such that, given a vector of n numbers, create all possible combinations from them (the same expand.grid() does, really).

For example, given the vector [1, 2] I want to create a function such that it outputs all possible combinations, i.e., "11", "12", "21", "21".

This is what I've come up with:

V <- trunc(runif(3, 0, 9))
FE2 <- function(V){
  C <- c()
  n <- 0
  for(i in 1:length(V)){
    for (j in 1:length(V)){
      for (k in 1:length(V)){
        C <- c(paste0(V[i], V[j], V[k]))
        n <- n   1
        print(c(paste0("Number of combination: ", n), paste0("Combination: ", C)))
      }
    }
  }
}
FE2(V)

The function does work. The problem is that for each element of the original vector, I have to add another for(). That is, if I wanted to compute all possible combinations for a vector of 10 elements, say [1, 2, ..., 10] I would have to create 10 for()-loops in my function. I wonder if there's another, more efficient way to do it, as long as it does not significantly differs from my actual solution.

CodePudding user response：

With expand.grid you then need to "collapse" the rows:

apply( expand.grid( 1:2, 1:2), 1, paste0, collapse="")
[1] "11" "21" "12" "22"

I'm not sure if I understood the goals but here's that method applied to 1:10 with 2 and then 3 instances of the vector.

> str(apply( expand.grid( 1:10, 1:10), 1, paste0, collapse="") )
 chr [1:100] "11" "21" "31" "41" "51" "61" "71" "81" "91" "101" "12" "22" "32" "42" "52" "62" "72" "82" "92" ...
> str(apply( expand.grid( 1:10, 1:10, 1:10), 1, paste0, collapse="") )
 chr [1:1000] "111" "211" "311" "411" "511" "611" "711" "811" "911" "1011" "121" "221" "321" "421" "521" "621" ...

CodePudding user response：

x <- 1:3

Use Reduce to iteratively apply the same operation

op <- function (a, b) {
  paste(rep(a, times = length(b)), rep(b, each = length(a)), sep = "-")
}

Reduce(op, rep(list(x), length(x)))
# [1] "1-1-1" "2-1-1" "3-1-1" "1-2-1" "2-2-1" "3-2-1" "1-3-1" "2-3-1" "3-3-1"
#[10] "1-1-2" "2-1-2" "3-1-2" "1-2-2" "2-2-2" "3-2-2" "1-3-2" "2-3-2" "3-3-2"
#[19] "1-1-3" "2-1-3" "3-1-3" "1-2-3" "2-2-3" "3-2-3" "1-3-3" "2-3-3" "3-3-3"

Generate permutations

library(RcppAlgos)

permuteGeneral(x, length(x), repetition = TRUE,
               FUN = function (x) paste0(x, collapse = "-"))